TBC - raids not being 20/40 man and JC. I like gemming my gear. /shrug
WOTLK - Dual spec. That was so needed.
Cata - Flying everywhere. Old world really needed the revamp.
MOP - Monk, Monk, Monk, and Monk. I love my monk so much...

I also agree that blizzard should not be praised for transmog. I recently saw they said they didn't even know how successful it would be. They are that dense. They completely ignored a feature everyone I know has wanted since Vanilla and other games had it already so all they had to do was copy it.

TBC: The content of Outland, the atmosphere and feeling of being in another world.

Wotlk: Nordic theme, I was a "fan" of nordic mythology as a kid and knew lots about it. Have to say seeing Freya and the rest as NPCs/bosses made me die on the inside a bit, though.

Cataclysm: Transmogrification.

MoP: Same as TBC, Pandaria is so beautiful.

I love the people here who say that quests are their favorite things in MoP! Most of the people I encounter hates it and everything that takes time in this game, I'm starting to think that about 90% of everyone I come across in this game has picked the wrong genre. They play an MMORPG but hates the RPG part.

There is actually a bank in the vale of eternal blossoms, I've used it a couple times. Transmog place too.

TBC: Server wide progressive daily questing area(Isle)
WOTLK: Showdown with LK on the top of the world and him actually killing you was so badass to me.
Cata: Questing and world revitalization.
MoP: So much to do...

The generalist looks outward; he looks for living principles, knowing full well that such principles change, that they develop. It is to the characteristics of change itself that the mentat-generalist must look. There can be no permanent catalogue of such change, no handbook or manual. You must look at it with as few preconceptions as possible, asking yourself, "Now what is this thing doing?"-Children of Dune

Classic: The game was new, and so everything was like ..... woah that is so awesome
TBC: Horde got paladins
WotLK: Wrathgate questline/event
Cataclysm: World revamp
MoP: so much to do, don't need to do all of it.

MoP - AOE looting. Sure you don't notice it now, but going from the beta to live without it made live feel dreadful.

I don't know what kind of ungrateful rat bastard doesn't notice aoe looting. I notice it every time I loot. Every single time I loot and loot more than one mob at once, I think, god that is so great that I don't have to loot all of them individually. Every time. I used to not even loot mobs, but now that I can loot them all at once, I loot them all the time. And, every time, I'm like, oh god that is awesome. Whenever I kill just one mob and loot it, I think, why didn't I kill more so that I could experience more amazing aoe looting? Whenever I see a mob I can loot in a dungeon, I loot it. Before, I never would, but now I know if there are any other loots I can loot, looting that one mob will loot them and that makes looting that one mob I see sparkling worth it.

I'm not kidding about every time. I'm aoeing stags in Jade Forest to skin on my alt right now and every time I loot, I think oh my god imagine if I had to loot each one of these individually.

Every expansion has had many good aspects, my list is just my personal favorites.

The Burning Crusade: Paladins became available to the Horde.
Wrath of the Lich King: Northrend, the art and design of the zones and instances was great.
Cataclysm: Reforging.
Mists of Pandaria: Account-wide mounts/achievements.

Also, I have to argue against flying mounts being a positive addition to the game. While they are convenient and make questing significantly easier, they remove the player from the game and shrink the world.